It is well known that to strengthen a muscle one should perform training involving heavy loads or resistance. Recently we have found that substantial voluntary strength gains can be achieved with training involving low resistance but strong mental effort. In contrast, individuals who trained with the same low-intensity contractions but with low mental effort had no improvement in strength. Based on these preliminary findings, we hypothesize that muscle strength improvements depend primarily on the level of mental effort during training, not the training intensity (resistance) per se. The reason that high-intensity training always increases strength is because mental effort is high during high-intensity muscle contractions. Aim 1 of the project is to compare the effects of training with different levels of mental effort on the improvement in muscle strength. Four groups of elderly subjects (greater than or equal to 65 years) will participate in a 12-week training study involving elbow-flexor muscles. One group will be trained with an intensity near the level of maximal voluntary contraction (MVC group); a second group will be trained with high mental-effort, low muscle-intensity elbow-flexion contractions (LME group); and the fourth (control) group will not be trained will participate in the strength tests. We expect that the strength improvement after training will be: MVC group > HME group > LME group = control group. We also expect that the strength increase in the MVC and HME groups will result in an improvement in daily living function. Aim 2 is determine the neural mechanisms underlying muscle strength improvements. We hypothesize that an increase in the central nervous system (CNS) drive is the primary mechanism that mediates strength improvements induced by low-intensity training (HME group). To evaluate the CNS drive, four measurements will be made using the same subjects and groups as in Ami 1: brain activation level examined by functional MRI (fMRI) and EEG-derived motor activity-related cortical potential (MRCP), surface EMG signals, and the MRI T2 relaxation time obtained from the trained muscles. We expect to find that after training: (1) the brain activation level (fMRI and MRCP), EMG, and MRI T2 will significantly increase in the MVC and HME groups; and (2) the amplitude of increases in these measurements will be: MVC group = HME group > LME group = control group. The knowledge gained from these studies will substantially advance the current understanding of mechanisms underlying human voluntary muscle strengthening and will have direct application in neuromuscular rehabilitation for older adults and individuals who are physically handicapped and unable to perform repeated, forceful muscle contractions.